1. Field
One embodiment of the invention relates to a video coding apparatus and a video coding method for determining one prediction mode from plural prediction modes including an intraprediction mode and an interprediction mode.
2. Description of the Related Art
H.261 and H.263 of International Telecommunication Union Telecommunication Standardization Sector (ITU-T) and MPEG-1, MPGE-2, MPGE-4 of Moving Picture Expert Group (MPGE) of International Organization for Standardization (ISO), etc., are developed as standard technologies of video coding. H.264 standardized by ISO and ITU jointly is a next-generation video coding method inherited from the technologies of H.261 to H.263, MPGE-1 to MPEG-4, etc., and further developed (See, for example, ITU-T Recommendation H.264 (2003), “Advanced Video Coding for generic audiovisual services” |ISO/IEC 14496-10: 2003, “Information technology, Coding of audio-visual objects—Part 10: Advanced video coding). However, H.264 involves a problem of a large processing amount although it achieves high data compression. Particularly, an encoder is required high computation processing capability to perform encode processing in real time in recording of digital broadcasting, etc.
The processing amount for determining a prediction mode in the entire coding processing is large to achieve video coding processing based on standardization specifications, etc., of H.264. Therefore, if the load on the apparatus is high, real-time coding processing is late. If a coded stream is played back, there is a possibility that defective conditions of dropping frames, extremely slow movement of an object, etc., may occur.
Particularly, for example, pixels in the same picture are referred and a prediction image is generated in High Profile of H.264 adopted in High Definition Digital Versatile Disk (HD DVD), etc. FIG. 10 shows details of intraprediction. The block size of 8×8 pixels can be used for the block size of prediction in High Profile in addition to the block size of 4×4 and 16×16 used in Main Profile. Therefore, the number of the prediction modes of intraprediction is 22 in total (nine modes of prediction block size 4×4 pixels, four modes of 16×16 pixels, and nine modes of 8×8 pixels).
FIG. 11 shows details of interprediction of referring pixels of pictures coded in the past and generating a prediction image. For each prediction block size of 8×8 pixels or more, which is the prediction image generation unit, the DCT block size (for DCT converting) of 8×8 pixels can be used in High Profile in addition to the block size of 4×4 pixels used in Main Profile. Therefore, if the prediction block sizes are limited to 16×16 pixels, 16×8 pixels, 8×16 pixels, and 8×8 pixels, the number of types of DCT block sizes is increased to two and thus the number of substantial prediction modes is increased from four to eight.
As the number of options of the prediction modes is increased as described above, the image quality of a video with a high resolution improves in H.264 High Profile. On the other hand, since a prediction mode is selected from among all prediction modes, the necessary processing amount increases with an increase in the number of the options of the prediction modes.
Particularly, in P picture and B picture for which the interprediction can be used, the prediction mode can be selected from both intraprediction and interprediction and thus the number of the options of the prediction modes is large. The P picture refers to a picture to refer pixels of one picture coded in the past in interprediction and generate a prediction image, and the B picture refers to a picture to refer pixels of two pictures coded in the past in interprediction and generate a prediction image.
Generally, the prediction accuracy of the interprediction is often higher than that of the intraprediction. The prediction mode of the interprediction is more likely to be selected for the P picture and the B picture. The total number of the prediction modes of the intraprediction is 22 and the total number of the prediction modes of the interprediction is eight as described above. Thus determination of the prediction mode of the intraprediction, which is less likely to be selected, requires a larger processing amount and the efficiency is very low.